It is predicted that mobile data usage will increase by 11-fold by 2018. To deal with this challenge, many new techniques have been proposed to improve data rate in the fifth generation (5G) wireless communication systems. However, the scarcity of the licensed spectrum for cellular networks is still the main bottleneck for further improvement of data rate. As a result, exploiting the unlicensed bands in small cells, currently used by WiFi, becomes a promising option.
WiFi is the most popular and successful technology to provide wireless service on unlicensed bands in a local area. With low cost and high data rate, WiFi systems already are the dominant player on all unlicensed bands in 2.4 GHz and 5 GHz. Most consumer electronic devices now come with a WiFi module. However, its spectrum efficiency is low when the network is overloaded.